It is known how to prepare fuel gases and particularly methane by anaerobic fermentation of masses consisting of agricultural, urban or even industrial organic wastes if they contain sufficient organic matter. In order that the production of the gases may be uninterrupted and the cost competitive, generally the operating methods applied must be continuous operating methods. A continuous process has still other very great advantages, such as, for example, reduction on idle time necessary for cleaning, filling, emptying etc . . . , reduction in the bulk of the equipment as a whole, great facility of operation, possibility of mechanization and automatization of the operations, and increase in the yield due to better choice of operating conditions.
Numerous methane-generating fermentation installations have been described; all the more encouraged by present increased energy needs and the shortage of conventional sources of energy based on petroleum stimulate researchers and producers to use this abundant and cheap raw material constituted by biomass.
Thus:
French Pat. No. 79 05662 describes a process for the preparation of methane from cow manure by anaerobic fermentation, a process that is characterized in that the substrate is introduced into a first preliminary treatment chamber where it is heated to a temperature on the order of 70.degree. C., then maintained at a temperature of about 55.degree.-60.degree. C. while being fed to a second treatment chamber and put in contact with a microorganism culture;
French Pat. No. 77 35966 describes an installation comprising a battery of three tanks arranged one above the other, the fermentation being aerobic in the first and anaerobic in the two others;
French Pat. No. 77 22386 describes a process for methane production by anaerobic fermentation of organic wastes in a fluid-tight fermentation tank completely filled with water;
French Pat. No. 75 09057 describes a digestion installation for organic matter by anaerobic fermentation, the installation comprising a vessel with two compartments communicating through their upper parts;
German Pat. No. 2 535 756 describes a production installation for making methane which consists of a horizontally inclined cylindrical tank provided with an axial stirrer;
U.S. Pat. No. 4,022,665 describes an anaerobic digestion process utilizing organic wastes for methane production; said process comprises two separate steps, a first step for the production of low molecular weight organic acids, and a second step for converting these acid compounds into methane.
Other patents and publications further relate to the preparation of methane by anaerobic fermentation.
Actually, digestion of organic wastes in an anaerobic process has been known for decades. Such a process uses mixed microbial cultures and various substrates. Very many microorganisms are capable of producing methane and they are universally present in nature (marshes, manure, retting of flax, decomposition of organic matter in the soil, degradation of cellulose, etc . . . ). However it has not been possible heretofore to perfect an economically profitable installation, which is sturdy and easy to operate and for use on any farm, which is the main supplier of raw material such as manure, liquid manure, straw, etc . . . . The reasons for this are very simple; and even explain why the solutions specified in the above-cited patents are no longer economically viable. They are principally the following:
Fermentation installations, particularly for continuous fermentation, which are known so far, operate in a dilute aqueous medium. These fermentations approximate the digestion and treatment of sewage, the optimal charge of which is around 8% of organic matter. In continuous fermentation installations, a stirrer or similar device is necessary to break the crust that is formed on the free surface, which serves as a surface for the release of gases produced (holding of the latter is dangerous for the installation). The transfer of the fluids and the stirring action require powerful energy-consuming pumps. Since the overall chemical reaction in this process is very slightly exothermic, the heating and maintenance of the temperature necessary in the mezophilic or thermophilic fermentation zones of the very large mass of water present also consumes much energy. By way of example, it is estimated that the treatment of a mud, having 2% fermentable matter, consumes almost all the methane produced which is used as a fuel to heat and maintain the temperature necessary for operation of the digester. In addition, in the majority of these continuous installations, the residue of the fermentation itself is also too aqueous for direct use as an organic fertilizer.